Conventionally, a process employing so called a through hole plating process, in which a through hole is created in a substrate by using a drill and a laser and a plated metal is precipitated on a wall of the through hole via an electroplating process, or a process for filling the through hole with a conductive paste of a material such as silver or copper via a screen printing process, which is often employed in general-purpose products, are utilized for achieving an electric coupling between a front side and a back side of a double-sided printed wiring board, which have electric conductor layers in both sides of the substrate member.
A process for filling with the conductive paste by the screen printing process is advantageous in terms of achieving better productivity. When silver is employed for the electric conductor, there is a problem of easily generating silver migration in higher temperature and higher humidity, though better electric coupling is provided.
On the other hand, when copper is employed as the electric conductor, generation of migration is inhibited, as compared with the case of employing silver, and therefore the use of the conductive pastes containing copper are spreading.
In addition, sophistications and miniaturizations of electronic devices lead to growing requirements in fine pitch-utilization and high electric conductivity-utilization in applications of the conductive pastes.
A mechanism of electric conduction in the conductive paste is mainly constituted with compression bonding between the electric conductor particles caused by cure shrinkage of a thermosetting resin serving as a binder. Therefore, the electroconductivity of the conductive paste is affected by a condition of oxidation of the surface of the electric conductor and a condition of compression of the binder resin.
For example, in the conventional technology, a technology for adding a chemical substance having reductive action to the copper paste is reported as a measure for preventing oxidation of copper surface (Patent Documents 1 and 2). In addition, in the electric conduction due to the compression bonding between the copper particles, which is the mechanism of electric conduction of the conductive paste, a presence of the oxide film on the surface of the copper powder serving as an insulating material is influential for an increase in the coupling resistance. Thus, an inhibition of an oxidation of the surfaces of the copper fine particles by employing a reducing agent is required (Patent Documents 3). However, it is difficult to reduce the electric conduction resistance in such approach, and thus sufficient characteristics for the requirements in the fine pitch-utilization and high electroconductivity-utilization in applications cannot be obtained.    Patent Document 1: Japanese Patent Laid-Open No. S61-3,154    Patent Document 2: Japanese Patent Laid-Open No. S63-286,477    Patent Document 3: Japanese Patent Laid-Open No. H08-73,780